Circuit for cathode-ray tubes and method of operating the same



Dec. 19, 1950 w. H. BUCHANAN 2,534,571

CIRCUIT FOR cATHQDE-RAY TUBES AND METHOD OF OPERATING THE SAME Filed June 24, 1948 2 Sheets-Sheet 1 I I IIIIIIIIIIIIIIIIIIIIIIII[III/{IIIIIII/ "III/1111111III/fii277D/I/II/ l3 v y n l2 INVENTOR. WILLIAM H. BUCHANAN A T TORNF X Dec. 19, '1950 Filed June 24, 1948 W. H. BUCHANAN CIRCUIT FOR CATHODE-RAY TUBES AND METHOD OF OPERATING THE SAME 2 Sheets-Sheet 2- TIME BASE IN V EN TOR.

' WILL/AM a BUCHANAN ATTORNEY Patented Dec. 19, 1950 UNITED STAT OFFHCE CIRCUIT FOR CATHGDE-RAY TUBES AND METHOD OF OPERATING THE SAME land Application June 24, 1948, Serial No. 35,038 In Great Britain October 7, 1947 16 Glaims.

This invention relates to improvements in circuits for cathode ra tubes and to methods of operating such tubes and is especially concerned with circuits for cathode ray tubes of the kind in which an electron beam of high intensity is produced by means of a thermionic cathode heated by bombardment with electrons.

Cathode ray tubes of this type comprise an anode, a cathode adapted to be heated by electron bombardment, a modulator electrode arranged to control the emission of electrons from said cathode towards said anode and a source of electrons for bombarding said cathode. Cathode ray tubes of this type will hereinafter be referred to as being of the type defined. Tubes of this type may, of course, contain also other electrodes such as additional anodes for focusing and acceleration of the electron beam and plates for the deflection of the beam in one or more directions, but these additional electrodes form no part of the present invention and are therefore ignored in this specification.

It is known that in cathode ray tubes in which an electron beam of high intensity is to be produced the use of a cathode heated by electron bombardment possesses several advantages. For example; whereas the heat input to any portion of a filamentary cathode increases as the cross section thereof decreases, with resultant accelerated diminution of cross section by evaporation and rapid failure of the cathode, the heat input to a bombarded cathode is proportional to the area exposed to bombardment so that failure from overheating occurs more rarely. cathodes of this type may therefore be constructed to operate at high temperatures, with consequent increased thermionic efiiciency and without the disadvantage of shortness of operating life which has hitherto accrued to high-temperature cathodes. A further advantage of bombarded cathodes is that the shape of the cathode is not influenced by the necessity to provide a return path for the heating current and, moreover, as the heating current is supplied to the cathode at high voltage and consequent small current, the connection to the cathode may be of small diameter, thus reducing the heat loss from end cooling.

However, in the operation of cathode ray tubes of this type, certain disadvantages have been found, which have limited the usefulness of such tubes and which it is an object of the present invention Wholly or in part to avoid.

These disadvantages arise from the fact that with many useful forms of cathode it is found dilhcult in practice to separate the bombarding electrons and secondary electrons emitted from the bombarded cathode surface from those emitted thermionically from the cathode. The result has been that the true electron stream from the cathode has been contaminated by the addition thereto of electrons having velocities widely different from those of the thermionically emitted electrons or originating from points separated from the true emissive surface by comparatively large distances. Both of these contaminations result in bad focus of the resulting electron beam.

The present invention seeks to avoid the above-described disadvantages by arranging that such contaminating electrons are not present in the electron beam emitted from the cathode during operative periods of the cathode ray tube.

According to the present invention there is provided a circuit arrangement including a cathode ray tube of the type defined wherein a source of cyclically varying unidirectional potential is connected between said source of bombarding electrons and said cathode so that in operation said cathode is periodically rendered positive with respect to said source.

According to a feature of the present invention the said source of unidirectionally varying potential comprises a source of alternating potential and a unidirectionally conducting device connected in series.

In the above statement the expression source of alternating potential is not to be understood as limited to a source of substantially sinusoidal potential, but is intended to include any source of potential such that the polarity of its terminals is periodically reversed.

Equally according to the present invention there is provided a method of operating a cathode ray tube of the type defined comprising applying between the cathode and the source of bombarding electrons, during part only of a cycle of operation to which said cathode ray tube is subjected, a potential such as to cause electrons emitted from said source to bombard said cathode.

According to a feature of this aspect of the invention, said cycle is one cycle of a scanning operation performed by the electron beam of said cathode ray tube.

In order that the present invention may be more readily understood, there follows a detailed description thereof with referen e being made to the accompanying drawings in which:

Fig. 1 is one embodiment of a circuit arrangement according to the invention; and

Fig. 2 is another embodiment of a circuit arrangement according to the invention.

In Fig. 1 there is shown part of a cathode ray tube l containing an anode 2, a cathode 3, which is arranged to be heated by bombardment with electrons from a helical filament 4 surrounding it, and a modulator electrode 5 surrounding one end of said cathode. These parts are shown in cross section so as to render their relative positions more readily visible. A source of high-tension potential (not shown) is connected between the anode and cathode so that electrons emitted from the latter are accelerated towards a part of the cathode ray tube in which the electron beam is utilized. The purpose for which and means whereby the said beam is utilized form no part of the present invention.

The helical filament 4 which serves as the source of bombarding electrons is heated by the passage therethrough of an electric current of suitable magnitude supplied from any convenient source of alternating current by means of a transformer 5, and a potential sufficient to accelerate the emitted electrons on to the cathode 3 is produced between the filament and cathode 3 by the connection therebetween of the secondary Winding of a transformer H, to the primary of which is applied an alternating potential derived from a circuit controlling cycles of operation of the cathode ray tube, in series with a thermionic diode valve 8 which is so connected that it substantially prevents the passage of electrons within the cathode ray tube in the direction from the cathode 3 to the filament l, but freely allows their passage in the reverse direction. The result of this arrangement is that the cathode 3 is bombarded by electrons from the filament l during such times as the potential derived from the transformer l is such as to maintain the cathode positive with respect to the filament, but that at all other times the cathode is not bombarded.

In the application of the present invention to a television system, the potential applied to the primary winding of the transformer "i may be derived, for example from the time-base generator which controls one component of the scanning operation, the sense of connection being such that the cathode is bombarded only during the fiyback intervals of the scanning process. This arrangement insures that only electrons emitted thermionically from the cathode can be utilized in the formation of the scanning electron beam, which is thus composed substantially wholly of electrons of nearly uniform velocity and of limited source of origin, whereby the focus of the electron beam is improved.

This arrangement may of course be adapted to other uses of the cathode ray tube, for example, in a radar receiving equipment the cathode may be arranged to be bombarded only dur-- ing the interval between the end of one stroke of the ranging time base and the commencement of the next stroke; the application of the invention to oscillographic uses of the cathode ray tube follows upon the same lines.

In some of these applications it may be found convenient to replace the transformer I by a source of cyclically varying unidirectional potential, the polarity of which is such that electrons are accelerated from the filament 4 to the cathode 3 during part of the cycle of variation of the said potential.

The circuit shown in Fig. 1 includes an arrangement which is found convenient in cases where modulation of the electron beam is effected during part of the operating cycle by means of signals applied to the modulator electrode 5 of the tube. The modulator electrode is shown connected by way of a high resistance 9 and a source of potential, represented by a battery It shunted by a potentiometer l l, to the cathode. Signals to be applied to the modulating electrode are applied to the terminal i2, whence they pass by way of condenser I3 to the modulating electrode 5, the resistance 9 being of sufliciently high value to have substantially no shunting effect thereon; this arrangement is, of course, well known. In carrying out the present invention, however, it is found advantageous that the modulator electrode shall, during those portions of the operating cycle in which bombardment of the cathode is carried on, be held negative with respect .to the cathode in order to suppress emission therefrom and at the same time shall not acquire a potential so negative that bombarding electrons are repelled from that part of the cathode which is adjacent to the modulator electrode.

These conditions are obtained, according to a feature of the invention, by the provision in shunt with resistance 9 of a thermionic diode M, the connections of which are such that the resistance 9 is substantially short-circuited for electrons passing from the modulator electrode towards the filament 4, by way of the battery l0, transformer l and diode 8. It will be seen that by this arrangement the electrons passing to the modulator electrode do not build up a negative charge thereon, as would be produced by the presence of the resistance 9 in the absence of the diode Hi, but pass freely back to the filament through the diode. The direction of current flow through the therefrom in the direction of the anode and to prevent the emission towards the anode of electrons from any source.

The modulating signal applied to the terminal i2 is preferably zero during heating portions of the operating cycle while during operating portions of the cycle it is such as to drive the modulator electrode 5 in the positive direction sufliciently to obtain the desired emission from the cathode 3.

An alternative method of suppressing emission from the cathode towards the anode during heating portions of the cycle is to modulate the potential applied to the anode, reducing this potential substantially, preferably to zero, during those times when the cathode is being heated.

Fig. 2 shows part of a cathode ray tube similar to that illustrated in Fig. l and in which similar parts have like numbers. In the circuit arrangement of Fig. 2, however, the modulator electrode 5 is connected to the cathode through resistors 9 and E8 in series for a purpose hereinafter to be described. 7

The electron beam produced by the system comprising cathode 3, modulator electrode 5 and anode 2 is focused by a magnetic field produced by a coil ll energized from a suitable source of current which is not illustrated. The beam is then deflected as required by the magnetic field of a pair of coils l5 and it which are supplied with an appropriately varying current from a time base I4 of any suitable type. The output of the time base is also applied to the primary winding of transformer I, so that the potential applied through diode 8 to the filament 4 to cause bombardment of the cathode 3 by electrons therefrom is always in synchronism with the deflection of the beam.

The connections of the transformer windings are so chosen that the cathode of the diode 8 is positive with respect to its anode; hence, the cathode of the tube is positive with respect to the source of bombarding electrons only during the flyback intervals of the scanning process performed by time base 14. By this means it is arranged that the cathode 3 is bombarded so that it is heated only during those times when the electron beam is not used to form the image on the screen of the cathode ray tube.

It is customary to suppress the electron beam of a cathode ray tube during the flyback of the scanning process. In the circuit illustrated in Fig. 2 this is done by applying across resistor 18 in the modulator electrode circuit a negativegoing pulse derived from the time base Hi. The value of resistor l 8 is chosen such that the modulator electrode potential is reduced to a point where electron emission from cathode 2 is substantially prevented.

Among the advantages of the present invention are that its use enables certain desirable constructions of cathode ray tube to be more conveniently used. For example, the shape of the cathode 3 may be varied from that shown so that the emissive portion is a sharp point, thus making use of the enhanced emission obtainable from such a cathode owing to the high field gradient obtaining in its neighborhood. A further advantage which may be obtained is that the cathode of a cathode ray tube operated in accordance with the invention may more readily be made of a form which is conveniently removed from the tube and replaced when its emission falls oil.

The invention is not, of course, limited to the specific embodiment illustrated in the drawing accompanying the specification as it may be carried out in various other ways as hereinbefore described or indicated.

What is claimed is:

1. A circuit comprising a cathode ray tube havin an anode, a cathode, a source of electrons for bombarding said cathode and a modulator electrode for controlling the emission of electrons from the cathode to the anode, and a source of cyclically varying unidirectional potential connected between said source of bombarding electrons and said cathode, whereby said cathode is periodically made positive with respect to said source of bombarding electrons.

2. The circuit according to claim 1, and in which said source of cyclically varying unidirectional potential comprises a source of alternating current and a substantially unidirectional conducting device connected in series.

3. The circuit according to claim 1, and in which said source of cyclically varying unidirectional potential comprises a time base circuit for deflecting the electron beam in said cathode ray tube.

4. The circuit according to claim 1, and in which a second substantially unidirectional conducting device is connected between said cathode and said modulator electrode whereby the electrons may pass through said device from said modulator electrode towards the cathode.

5. The circuit according to claim 1, and means for varying the potential between the cathode and the modulator electrode so as substantially to prevent the emission of electrons from said cathode in the direction of the anode during bombardment of the cathode.

6. The circuit according to claim 1, and means for varying the potential between the cathode and the anode so as substantially to prevent the emission of electrons from said cathode in the direction of the anode during bombardment of the cathode.

7. The method of operating a cathode ray tube comprising the following steps: feeding to deflection means of the tube cyclical sweeping voltages, and bombarding the cathode with electrons in synchronism to the sweep voltages during predetermined portions of cycles thereof to heat the cathode.

8. The method according to claim '7, and in which said portions of the cycles are the flyback portion thereof.

9. The method of operating a cathode ray tube of the type defined comprising the following steps: feeding to deflection means of the tube cyclical sweeping voltages, bombarding the oathode with electrons in synchronism to the sweep voltages during the flyback portions thereof, and cyclically varying the potential between the modulator electrode and the cathode so as to prevent electron emission from the cathode towards the anode during bombardment of the cathode.

10. The method of operating a cathode ray tube of the type defined compriisng the following steps: feeding to deflection means of the tube cyclical sweeping voltages, bombarding the cathode with electrons in synchronism to the sweep voltages during the flyback portions thereof, and cyclically varying the potential between the anode and cathode so as to prevent emission from the cathode towards the anode during bombardment of the cathode.

WILLIAM HUDSPITH BUCHANAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,864,591 Foster June 28, 1932 2,197,033 Diels Apr. 16, 1940 2,207,276 Somers July 9, 1940 2,239,416 Ehrenberg Apr. 22, 1941 2,355,795 Glass Aug. 15, 1944 2,398,829 Haefi Apr. 23, 1946 

